Sequence control of an industrial process in accordance with a relay logic diagram lends itself to digital control and with certain techniques such as shown in the referenced copending application offers a remarkable degree of sophistication with a minimun capacity requirement and an attractive low cost configuration. When it comes to numerical control, a digital handling of numerial data because it requires conversion of data from decimal to binary and involves mathematical treatment rather than mere Boolean equations, introduces elements of complexity which upset the favorable aspects of sequence control. As a result the prior art generally shows numerical control machines which combine the two aspects of the control operation and such equipment no longer qualifies as a sequence controller, and even less as a programmable controller.
It is also known to have a digitally controlled incremental system for imparting point-to-point movement to a machine tool activated by a stepping motor. Typical of such prior art are U.S. Pat. No. 3,324,281 of Morse and United States Pat. No. 3,403,386 of Perkins. However, the prior art devices do not make a full use of digital techniques. Thus, Morse and Perkins depend too much on mechanical devices. More generally they do not lend themselves to full digital treatment nor to an extensive use of integrated circuit technology. In particular, sequence control is not harmoniously combined with motion control.
Sequence controllers are well known, and an overview of the state of the prior art in this respect can be found in "Programmable Logic Controllers-An Update" by N. Andreiev, in Control Engineering of Sept., 1972, pages 45-47; and in "Programmable Logic Controllers-Painless Programming to Replace the Relay Bank" by G. Lapidus in Control Engineering of Apr., 1971, pages 49-60. On the other hand, the art of computerized numerical control is largely described in "The Technical Ins and Outs of Computer-ized Numerical Control" by P. G. Mesniaeff in Control Engineering of Mar., 1971, pages 65-84.
The chief object of the present invention is to provide a harmonious combination of sequence controlling and numerical control.
Another object of the present invention is to provide a motion control unit which can be coupled to a programmable controller for point-to-point operation along an axis.
A further object of the present invention is to extend the capability of a programmable controller so as to perform numerical control operations without affecting the structural logic of a programmable controller.
Still another object of the present invention is to provide improved motion control of a machine tool with extensive use of printed circuit technology.
An object of the present invention is also to provide improved motion control of a grinding machine, in particular with automatic compensation for dressing operations of the grinding wheel.